The 5° Diameter Ionized Halo of the Planetary Nebula Abell 36

We have observed an ionized halo surrounding the planetary nebula Abell 36. It is barrel-shaped, with dimensions 4 degrees x 5 degrees, which is 17 pc x 21 pc at a distance of 240 pc. With an average H alpha surface brightness of 0.8 +/- 0.2 rayleighs, the halo's total H alpha flux, 8.3 x 10(-10) ergs cm(-2) s(-1), exceeds that of the previously known inner part of the nebula by a factor of 30. The ionized mass of the halo is 48 root epsilon M ., where epsilon is the filling factor of the ionized gas. Velocity-resolved Ha spectra indicate that the halo is ambient interstellar matter and not matter ejected from the planetary nebula. The edges of the halo are evident in both 100 mum emission and red optical continuum.